Electrochromic indicators employ solid inorganic electrochromic materials. An electrochromic material is responsive to an application of an electric field of a given polarity so that it changes from a first persistent state, in which it is essentially non-absorptive of electromagnetic radiation in a given wavelength region, to a second persistent state in which it absorbs electromagnetic radiation. Once in said second state, such a material is responsive to an application of an electric field of the opposite polarity to return to its first state. Electrochromic materials include oxides, sulfides and some other compounds of transition metals, and are dealt with in U.S. Pat. Nos. 3,521,941 and 3,819,252, Cl. 350-357. The electrochromic effect is observed in thin films of oxides of transition metals, such as tungsten oxide, after a double injection into the film of an electrochromic material of electrons and protons (or ions of alkali metals), whereby colored compounds are produced, such as tungsten bronze if use is made of tungsten oxide. Protons (or ions of metals) come from the electrolyte. The mechanism of electrochromic effect in oxides of transition metals is described by F. Chang (cf. F. Chang, Proceedings of SID, 1975, vol. 16/3, pp. 168-177).
Coloration of an electrochromic film normally occurs when the film is placed between two electrodes whereof at least one is transparent. In order to provide for reversible operation, i.e. alternating coloration and bleaching, the electrochromic layer is separated from one of the electrodes by a layer of a material which possesses ionic conductivity and serves as a barrier against the transfer of electrons.
Electrically induced coloration of films of WO.sub.3 is believed to be due to the formation of tungsten bronze in the course of this reaction: EQU xe+xA.sup.+ +WO.sub.3 .revreaction.A.sub.x WO.sub.3 xe, (1)
where
x is a positive integer,
A.sup.+ is a positive ion, for example, H.sup.+, Na.sup.+, Li.sup.+, K.sup.+,
e is an electron,
WO.sub.3 is a colorless film, and
A.sub.x WO.sub.3 xe is a colored film.
Electrons are injected into the film of WO.sub.3 from an adjacent electrode: this means that the electrode adjoining the film of WO.sub.3 must be negative to induce coloration. Positive ions arrive from the ionic conductivity layer which serves as a barrier against the electron transfer. As electrons and positive ions are brought together, they produce compounds of the A.sub.x.sup.+ WO.sub.3 xe type, which have coloration centers due to the localization of electrons on ions of tungsten. In this case the process (1) proceeds from left to right. With a reversal of polarity of the applied voltage, i.e. when the electrode adjacent to the film of WO.sub.3 becomes positive, electrons are forced out of the film of WO.sub.3, and the latter is bleached. The advent of electrons to the film of WO.sub.3 from the other electrode is prevented by the barrier layer. In this case the process (1) proceeds from right to left.
Several types of electrochromic indicators have been developed recently which feature improvements in certain working parameters and better reliability, as compared to the earlier types. However, none of the existing electrochromic indicators can boast of sufficiently long service life because of a number of irreversible chemical and electrochemical processes occurring in such indicators.
There is known an electrochromic indicator (cf. U.S. Pat. No. 3,521,941, Cl. 350-357) which is a multilayer structure comprising in a sandwich layered arrangement an optically transparent substrate, an optically transparent first electrode, a layer of a solid inorganic electrochromic material, an insulating layer, and a second electrode. The indicator may also include a second substrate.
In the electrochromic indicator under review, the insulating layer is a thin film of calcium fluoride which absorbs moisture, carbon dioxide, ammonia and other gases from the environment. The water-soluble gases serve as an electrolyte containing cations which are necessary for electrically induced coloration of the electrochromic layer. The indicator lacks a permanent source of cations, so the supply of cations is dependent upon the state of the surrounding medium. Indicators of this type can only live through a limited number of coloration-bleaching cycles, and their response in inadequate.
From the standpoint of technical essence, the closest prototype of the present invention is the electrochromic indicator disclosed in U.S. Pat. No. 3,944,333, Cl. 350-357. This indicator is a multilayer structure comprising in a sandwich layered arrangement an optically transparent first substrate, an optically transparent first electrode, a layer of a solid inorganic electrochromic material, a layer of an electrolyte, a second electrode, and a second substrate.
The electrolyte is sulfuric acid contained in pores of a thin film of a material which is chemically inert to sulfuric acid.
As stated above, the electrolyte is a source of cations in an electrochromic indicator; as a second-class conductor, the electrolyte also prevents the transfer of electrons from the second electrode to the electrochromic layer.
According to common belief, the principle of operation of the foregoing types of electrochromic indicators is as follows. As electric current is passed from the second electrode to the transparent first electrode, cations and electrons are injected into the electrochromic layer to produce coloration centers therein. Reversal of the current makes the coloration centers disintegrate, and the electrochromic layer is bleached. To ensure reversible operation of an electrochromic indicator, i.e. an alternation of coloration and bleaching, one must meet certain conditions. First, the transparent electrode is expected to serve as a source of electrons, as well as to provide for a transfer of electrons in the opposite direction. Second, the electrolyte must serve as a source of cations and provide for a transfer of cations in the opposite direction.
The indicator according to U.S. Pat. No. 3,944,333, Cl. 350-357, satisfies these requirements. At the same time it has certain disadvantages. As electric current is passed through the indicator, irreversible electrochemical reactions are initiated on the boundary between the electrolyte and the second electrode. Products of these reactions, including peroxides, accumulate in the indicator and destroy the second electrode; they also affect the composition of the electrolyte and the electrochromic layer. Eventually the indicator is rendered inoperative.
The electrolyte of the indicator under review is such that cations, including ions of hydrogen, are produced therein due to the electrolytic dissociation of the acid. The transfer of anions to the second electrode is of the diffusive and electric types. On the surface of the second electrode, anions enter into electrochemical reactions and accumulate in the electrolyte as reduced or oxidized compounds. When strong acids are used, anions are diffused throughout the entire volume of the indicator and may cause chemical decomposition of the second electrode and the electrochromic layer. Anions may also be the cause of a depressurization of the indicator. Significantly, this may happen during storage, i.e. before the indicator is put into operation. All these factors affect the reliability and account for short life of the indicator.